It’s been a rollercoaster few months in the world of nutrition science – which is saying something, given the wildly contradictory claims emanating from that field as a matter of course. (You know the sort of thing: will fish oil or dark chocolate or a daily glass of wine keep you healthy to 100, or drive you to an early grave?) Now, a big US study has found that low-carb diets could shorten life expectancy by up to four years. Meanwhile, there’s a crisis with PrediMed, the landmark study purporting to show that a “Mediterranean” diet – fish, nuts and vegetables, slathered in olive oil – could cut heart disease risk by a third. It turns out it wasn’t properly randomised, throwing that finding into doubt.

Is this “the end of nutrition science” as we know it, as vox.com speculated in June? And more urgently, since it’s late morning as I write this, what the hell am I supposed to have for lunch?

There’s a less histrionic way to think about all this, though. Read that first study carefully and you’ll find that the funeral-hastening effects of a low-carb regime aren’t really an issue, so long as you replace the carbs with plant-based foods. And the truth about the Mediterranean diet is almost certainly that it’s good, just not as astonishingly good as some thought. (Reducing heart disease risk by a third would be an improbably huge result for any diet.)

In other words, far from throwing anything into doubt, these studies add weight to what you already knew: you should eat plenty of vegetables, not too much meat, very little processed junk or added sugar, and stop before you’re stuffed. Or, in the famous summation of the author Michael Pollan: “Eat food, not too much, mostly plants.”

This is the real oddity of nutrition science: not simply that nobody knows anything, but that, somehow, nobody knows anything and we already know all we need to know. There’s a good public health argument for studying the details. But on a personal level, unless you’re already doing a perfect job of eating a high-veg, moderate-meat, low-junk diet, that’s what to focus on. No fiddling with the technicalities could possibly have so much impact.

And it’s not just nutrition, either. You already know how to exercise (move around a lot, sometimes near the limits of your capacities) and how to manage your time (protect time for the most important stuff, preferably first thing, and stay off social media). You know how to date (put yourself in social situations repeatedly, and don’t be an arse) and how to nurture a relationship (listen hard).

We forget we know all this, in part, because of an advice industry that fixates on exciting new details, in order to sell its wares. But it’s our fault, too: we crave novelty, and the old advice seems boring. And so we become what the satirist of business life Laurence Peter called “side-issue specialists”, whose motto is “look after the molehills, and the mountains will look after themselves”.

In truth, though, you’ve no right to dismiss the old advice as boring if you haven’t implemented it yet. It’s old for a reason: so far, nobody’s managed to kill it.

oliver.burkeman@theguardian.com

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Triumphs Of Experience, by George Vaillant, recounts the story of the Harvard Grant Study, which tracked the wellbeing of 268 men over the course of 75years – an approach that enabled researchers to sift the factors fundamental to their health and happiness, discarding irrelevant details.